Try to build with GCC's -Wdeclaration-after-statement.

This will help catch things that'll cause issues on C89 compilers before we
send them on to fail on Buildbot.

CMakeLists.txt

@@ -334,6 +334,24 @@ set(HAVE_ASSERTIONS ${ASSERTIONS})
 
 # Compiler option evaluation
 if(USE_GCC OR USE_CLANG)
+  # Check for -Wall first, so later things can override pieces of it.
+  check_c_compiler_flag(-Wall HAVE_GCC_WALL)
+  if(HAVE_GCC_WALL)
+    list(APPEND EXTRA_CFLAGS "-Wall")
+    if(HAIKU)
+      set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wno-multichar")
+    endif()
+  endif()
+
+  check_c_compiler_flag(-Wdeclaration-after-statement HAVE_GCC_WDECLARATION_AFTER_STATEMENT)
+  if(HAVE_GCC_WDECLARATION_AFTER_STATEMENT)
+    check_c_compiler_flag(-Werror=declaration-after-statement HAVE_GCC_WERROR_DECLARATION_AFTER_STATEMENT)
+    if(HAVE_GCC_WERROR_DECLARATION_AFTER_STATEMENT)
+      list(APPEND EXTRA_CFLAGS "-Werror=declaration-after-statement")
+    endif()
+    list(APPEND EXTRA_CFLAGS "-Wdeclaration-after-statement")
+  endif()
+
   if(DEPENDENCY_TRACKING)
     check_c_source_compiles("
         #if !defined(__GNUC__) || __GNUC__ < 3
@@ -375,13 +393,6 @@ if(USE_GCC OR USE_CLANG)
   endif()
   set(CMAKE_REQUIRED_FLAGS ${ORIG_CMAKE_REQUIRED_FLAGS})
 
-  check_c_compiler_flag(-Wall HAVE_GCC_WALL)
-  if(HAVE_GCC_WALL)
-    list(APPEND EXTRA_CFLAGS "-Wall")
-    if(HAIKU)
-      set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wno-multichar")
-    endif()
-  endif()
   check_c_compiler_flag(-Wshadow HAVE_GCC_WSHADOW)
   if(HAVE_GCC_WSHADOW)
     list(APPEND EXTRA_CFLAGS "-Wshadow")

configure

@@ -18650,6 +18650,43 @@ $as_echo "$have_gcc_preferred_stack_boundary" >&6; }
     fi
 }
 
+CheckDeclarationAfterStatement()
+{
+    { $as_echo "$as_me:${as_lineno-$LINENO}: checking for GCC -Wdeclaration-after-statement option" >&5
+$as_echo_n "checking for GCC -Wdeclaration-after-statement option... " >&6; }
+    have_gcc_declaration_after_statement=no
+
+    save_CFLAGS="$CFLAGS"
+    CFLAGS="$save_CFLAGS -Wdeclaration-after-statement -Werror=declaration-after-statement"
+    cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+
+    int x = 0;
+
+int
+main ()
+{
+
+
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_c_try_compile "$LINENO"; then :
+
+    have_gcc_declaration_after_statement=yes
+
+fi
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+    { $as_echo "$as_me:${as_lineno-$LINENO}: result: $have_gcc_declaration_after_statement" >&5
+$as_echo "$have_gcc_declaration_after_statement" >&6; }
+    CFLAGS="$save_CFLAGS"
+
+    if test x$have_gcc_declaration_after_statement = xyes; then
+        EXTRA_CFLAGS="$EXTRA_CFLAGS -Wdeclaration-after-statement -Werror=declaration-after-statement"
+    fi
+}
+
 CheckWarnAll()
 {
     { $as_echo "$as_me:${as_lineno-$LINENO}: checking for GCC -Wall option" >&5
@@ -22894,6 +22931,8 @@ fi
 
 }
 
+CheckWarnAll
+
 case "$host" in
     *-*-linux*|*-*-uclinux*|*-*-gnu*|*-*-k*bsd*-gnu|*-*-bsdi*|*-*-freebsd*|*-*-dragonfly*|*-*-netbsd*|*-*-openbsd*|*-*-sysv5*|*-*-solaris*|*-*-hpux*|*-*-aix*|*-*-minix*)
         case "$host" in
@@ -22962,6 +23001,7 @@ case "$host" in
             *-*-minix*)         ARCH=minix ;;
         esac
         CheckVisibilityHidden
+        CheckDeclarationAfterStatement
         CheckDummyVideo
         CheckDiskAudio
         CheckDummyAudio
@@ -23392,6 +23432,7 @@ $as_echo "#define SDL_FILESYSTEM_HAIKU 1" >>confdefs.h
         ARCH=ios
 
         CheckVisibilityHidden
+        CheckDeclarationAfterStatement
         CheckDummyVideo
         CheckDiskAudio
         CheckDummyAudio
@@ -23461,6 +23502,7 @@ $as_echo "#define SDL_FILESYSTEM_HAIKU 1" >>confdefs.h
         EXTRA_CFLAGS="$EXTRA_CFLAGS -DTARGET_API_MAC_OSX"
 
         CheckVisibilityHidden
+        CheckDeclarationAfterStatement
         CheckDummyVideo
         CheckDiskAudio
         CheckDummyAudio
@@ -23581,6 +23623,7 @@ $as_echo "#define SDL_AUDIO_DRIVER_EMSCRIPTEN 1" >>confdefs.h
         fi
 
         CheckVisibilityHidden
+        CheckDeclarationAfterStatement
         CheckDummyVideo
         CheckDiskAudio
         CheckDummyAudio
@@ -23630,8 +23673,6 @@ $as_echo "#define SDL_TIMER_UNIX 1" >>confdefs.h
         ;;
 esac
 
-CheckWarnAll
-
 # Verify that we have all the platform specific files we need
 
 if test x$have_joystick != xyes; then

configure.in

@@ -1124,6 +1124,30 @@ CheckStackBoundary()
     fi
 }
 
+dnl See if GCC's -Wdeclaration-after-statement is supported.
+dnl  This lets us catch things that would fail on a C89 compiler when using
+dnl  a modern GCC.
+CheckDeclarationAfterStatement()
+{
+    AC_MSG_CHECKING(for GCC -Wdeclaration-after-statement option)
+    have_gcc_declaration_after_statement=no
+
+    save_CFLAGS="$CFLAGS"
+    CFLAGS="$save_CFLAGS -Wdeclaration-after-statement -Werror=declaration-after-statement"
+    AC_TRY_COMPILE([
+    int x = 0;
+    ],[
+    ],[
+    have_gcc_declaration_after_statement=yes
+    ])
+    AC_MSG_RESULT($have_gcc_declaration_after_statement)
+    CFLAGS="$save_CFLAGS"
+
+    if test x$have_gcc_declaration_after_statement = xyes; then
+        EXTRA_CFLAGS="$EXTRA_CFLAGS -Wdeclaration-after-statement -Werror=declaration-after-statement"
+    fi
+}
+
 dnl See if GCC's -Wall is supported.
 CheckWarnAll()
 {
@@ -2801,6 +2825,9 @@ AC_HELP_STRING([--enable-rpath], [use an rpath when linking SDL [[default=yes]]]
                   , enable_rpath=yes)
 }
 
+dnl Do this on all platforms, before everything else (other things might want to override it).
+CheckWarnAll
+
 dnl Set up the configuration based on the host platform!
 case "$host" in
     *-*-linux*|*-*-uclinux*|*-*-gnu*|*-*-k*bsd*-gnu|*-*-bsdi*|*-*-freebsd*|*-*-dragonfly*|*-*-netbsd*|*-*-openbsd*|*-*-sysv5*|*-*-solaris*|*-*-hpux*|*-*-aix*|*-*-minix*)
@@ -2870,6 +2897,7 @@ case "$host" in
             *-*-minix*)         ARCH=minix ;;
         esac
         CheckVisibilityHidden
+        CheckDeclarationAfterStatement
         CheckDummyVideo
         CheckDiskAudio
         CheckDummyAudio
@@ -3196,6 +3224,7 @@ AC_HELP_STRING([--enable-render-d3d], [enable the Direct3D render driver [[defau
         ARCH=ios
 
         CheckVisibilityHidden
+        CheckDeclarationAfterStatement
         CheckDummyVideo
         CheckDiskAudio
         CheckDummyAudio
@@ -3265,6 +3294,7 @@ AC_HELP_STRING([--enable-render-d3d], [enable the Direct3D render driver [[defau
         EXTRA_CFLAGS="$EXTRA_CFLAGS -DTARGET_API_MAC_OSX"
 
         CheckVisibilityHidden
+        CheckDeclarationAfterStatement
         CheckDummyVideo
         CheckDiskAudio
         CheckDummyAudio
@@ -3366,6 +3396,7 @@ AC_HELP_STRING([--enable-render-d3d], [enable the Direct3D render driver [[defau
         fi
 
         CheckVisibilityHidden
+        CheckDeclarationAfterStatement
         CheckDummyVideo
         CheckDiskAudio
         CheckDummyAudio
@@ -3407,9 +3438,6 @@ AC_HELP_STRING([--enable-render-d3d], [enable the Direct3D render driver [[defau
         ;;
 esac
 
-dnl Do this on all platforms, after everything else.
-CheckWarnAll
-
 # Verify that we have all the platform specific files we need
 
 if test x$have_joystick != xyes; then